Composition analysis of a gas sample by Raman spectroscopy allows the measurement of the abundance of the chemical species within the gas sample for those chemical species composed of at least two atoms. Species composed of just one atom, such as the noble gases helium and argon, and ionic-bonded substances like salts, are invisible to Raman spectroscopy. In practice, such Raman-invisible species are present in some chemical mixtures and often need to be quantified.
Absorption spectroscopy such as near infrared spectroscopy and infrared spectroscopy suffer similar limitations as the Raman analysis. That is, substances such as homonuclear diatomics (O2, N2, etc.) and ionic-bonded salts are invisible to the absorption spectroscopy, but nevertheless these substances are present in some chemical mixtures and often need to be quantified.
In sample streams containing a component invisible to Raman or absorption spectroscopy, it is often necessary to supplement the spectroscopy measurements to quantify invisible species and obtain full composition analysis. For example, in the synthesis loop in a fertilizer plant argon is present along with other gases. Though Raman spectroscopy can characterize most of the gases in a sample, the detection of argon requires another technology such as gas chromatography or mass spectrometry. These methods are relatively expensive and not well suited to in situ detection within a pipeline or a reactor.
Less costly analysis methods exist which can quantify the Raman-invisible species, but such methods work well for mixtures with only a few components, ideally mixtures containing only two gases, i.e., binary mixtures. But in many industrial applications many gases will be present in a sample, and a direct application of methods for solving binary mixtures must be modified, and assumptions must be made regarding which components are changing and which are stable. Accordingly, there remains a need for further contributions in this area of technology to enable compositional analysis of mixtures that include spectroscopic-invisible species.